The present invention relates to the production, from fresh black or white grapes, of grape tannin from the marc or seed, containing a high content of polyphenolic compounds, the tannin thus obtained and the uses thereof especially in oenology.
Tannins are substances of plant origin which consist of polyphenolic compounds and which are divided into two main groups, catechuic tannins also called condensed or procyanidin tannins, to which the grape tannins belong, and the hydrolysable tannins predominantly comprising gallotannins and ellagitannins. Grape tannins are naturally present in grape skins and seeds.
These two groups are distinguishable by the nature of the polyphenolic compounds which they contain. The catechuic tannins resemble catechuic derivatives and procyanidolic oligomers and polymers consisting of 3-flavanol and 3,4-flavanediol units. More particularly, grape seed and skin tannins consist of catechin, epicatechin and epicatechin-3-O-gallate, the skin tannin containing, in addition, epigallocatechin which is absent from seed tannin. The polymers of skin tannin are longer (chains of up to 80 units) than the polymers of seed tannin (chains not exceeding 30 to 35 units). Finally, there are grafted on seed tannin molecules of gallic acid which is only present in very small quantities in skin tannin.
Gallotannins of the hydrolysable tannin group consist of polymers of glucose and of gallic acid and the ellagitannins of the same group consist of a polymer of glucose and of ellagic, gallic and/or hexahydroxydiphenic acids.
In oenology, tannins are used for the clarification of wines because they exhibit a strong affinity to bind to proteins. They are also used for improving the organoleptic qualities and the taste characters of wines.
Tannins extracted from chestnut tree, oak tree and nut galls of various trees, which are hydrolysable tannins, are mainly used.
The application of tannins is not limited to oenology, but it extends to many other sectors such as the tanning, agri-food, pharmaceutical, cosmetic, ceramic and textile sectors and the like.
Grape tannins are not used on an industrial scale in the manufacture of wines because of the low content of phenolic compounds in the extracts obtained. Indeed, the methods currently used for preparing grape tannins cause substantial degradation of the said compounds.
Despite the small proportion of polyphenolic compounds which they contain, grape tannins, and more particularly grape seed tannins, are used in the pharmaceutical sector as a medicament in particular for treating venous insufficiency-related disorders. The active ingredient is the fraction of procyanidolic oligomers contained in the tannin. However, because of the high degradability of these tannins, the purity, in relation to the said oligomers, of the medicaments currently available does not exceed 20%.
The method of production which is conventionally followed for obtaining tannins, grape tannins for example, consists in: (a) carrying out, starting with fresh black and/or white grape marc and/or seed, a solid-liquid extraction of a crude tannin fraction in an aqueous solvent, (b) removing the aqueous solvent from the resulting extract in order to obtain a concentrate of the crude tannin fraction, and (c) purifying the crude tannin fraction in order to obtain the said tannin, by a liquid-liquid extraction, evaporation of the solvent in which the tannin in solubilized and then filtration on activated charcoal.
According to the present invention, there is provided a method for producing a tannin having a high content of polyphenolic compounds, the said method incorporating defined stages such that they make it possible to preserve the integrity of the said compounds.
The method of the invention is characterized in that, in combination, according to stage (a) sulphited water (H2O+SO2) is used as solvent, and according to stage (c) the crude tannin fraction is purified by selective adsorption of the polyphenolic compounds of the tannin on resin and subsequent filtration.
Before disclosing the present invention in greater detail, various terms used in the description and the claims are defined below.
The grape marc comprises the grape stalk, that is to say the bunch without its grapes (peduncle and pedicels) and the skin which constitutes the envelope of the grape berry.
Grape marc tannin is therefore understood to mean the tannin obtained by treating the grape marc as defined above. However, given that the substances constituting the marc tannin are in fact obtained mainly from the grape skin, this expression ought to more precisely be understood to mean a grape skin tannin. In oenology, the marc is in fact a portion of the grapes harvested for winemaking, of white wine only, which is recovered after pressing the grape juice and then separating out the seeds. Accordingly, it generically designates the skin. Thus, according to the method of the invention, when the starting material is fresh grape marc, it will be understood that this may be the skin of fresh grape alone.
Fresh grape is understood to mean according to the invention a grape which has not been subjected to alcoholic fermentation or acetic fermentation.
The nature of the total phenolic compounds varies according to the type of tannin, as specified above. Their content is however always determined as equivalent gallic acid, according to the tannic acid technique of the international oenological codex.
Using the method of the invention, a tannin is obtained which comprises in particular gambiriines which are dimers. The tannin from the skin and the stalk according to the method described above comprises, in addition, resveratrol. The tannin from the skin also comprises various flavonols such as quercetin and kaempferol.
Advantageously, the method is defined by the following additional characteristics, taken in combination or otherwise:
stage (a) is carried out at a temperature at most equal to 25xc2x0 C., and preferably of between 15 and 25xc2x0 C.,
stage (b) is carried out by evaporation under vacuum at a temperature at most equal to 75xc2x0 C.,
after evaporation according to stage (b), the concentrate of the crude tannin fraction is immediately cooled to a temperature at most equal to 30xc2x0 C.,
the method comprises a stage (bxe2x80x2), between stages (b) and (c), according to which a fermentation of the concentrate of the crude tannin fraction is carried out; for the purposes of stage (bxe2x80x2), yeasts in suspension in a liquid medium are inoculated into the concentrate of the crude tannin fraction; the fermentation is allowed to proceed; the alcohol formed is removed followed by the particles in suspension in order to obtain a concentrate of the fermented crude tannin fraction.
The objective of the fermentation is the conversion of sugars to alcohol as well as the beneficial conversion of the tannins. This conversion makes the tannins conform to the organoleptic requirements of wines, because they undergo the same process of conversion as the intrinsic tannins of the wine.
according to stage (c), the crude tannin fraction is adsorbed on a resin, of the preferably microcrosslinked styrene divinylbenzene (DVB) type,
according to stage (c), the crude tannin fraction treated on resin is filtered by diafiltration, in particular selective diafiltration; the latter is advantageously carried out at a temperature at most equal to 25xc2x0 C.
Other subjects of the invention are:
the black and/or white grape marc tannin which can be obtained by the method of the invention, comprising a content of total polyphenolic compounds of at least about 300 mg equivalent gallic acid per g of dry matter, and
the black and/or white grape seed tannin which can be obtained by the method of the invention, comprising a content of total polyphenolic compounds of at least about 500 mg equivalent gallic acid per g of dry matter.
The invention relates, in addition, to the use of a white grape marc tannin as defined above, as endogenous tannin in wine. It also relates to the use of a grape tannin of the invention for producing an extract rich in procyanidolic oligomers.